Changes in the function and structure of chromatin and in the synthesis of non-histone chromosomal proteins have been demonstrated to occur in a number of situations in which quiescent cells are stimulated to proliferate. However, a rigorous correlation between the occurrence of these two phenomena and the extent of cell proliferation has not yet been established. To obtain a better demonstration that these events are, in fact, interrelated we propose to investigate the function and structure of chromatin and the synthesis of non-histone chromosomal proteins in temperature-sensitive mutants of BKH cells. The temperature sensitive mutants selected for these experiments are tsAF8 cells which have a cell cycle specific block located in the very early part of the G1 period. By investigating cell populations that are resting, or that have been stimulated at either the permissive, or non-permissive temperature, it should be possible to establish which events, located in the G1 period after the block, are necessary for the orderly flow of cells into DNA synthesis and mitosis. In addition it should be possible to develop hybird clones of these temperature sensitive tsAF8 cells with SV-40 transformed Lesch-Nyhan fibroblasts, hybrid clones that will be capable of growing in HAT medium at the temperature non-permissive for tsAF8. These hybird clones should contain human chromosomes that allow the growth of tsAF8 hamster cells at the non-permissive temperature. Immunological studies can be carried out then on the chromosomal proteins that are bound to the human chromosomes in the hybrid clones.